The purpose of the proposed work is to understand the cells involved in resistance to infection by intracellular bacteria. The model system uses mice infected with Listeria monocytogenes as a prototype for infection by an intracellular organism. Previous experiments suggest that bone marrow-derived cells in addition to and including the macrophage may be important in resistance to infection. Experiments are proposed to develop an in vitro system for quantitating Listeria infection. Using this system, critical studies of cells which mediate reistance, and the mechanism by which they do so can be clarified. Other experiments-past, present and future-are concerned with other biologic and immunologic effects of purified materials derived from Listeria monocytogenes.